The gnathal or post-antennal segments at first bear but a small part in completing the tergal region of the head, but shortly before hatching the mandibles and their muscles enlarge, giving fulness to the upper and back part of the head.

i. Dorsal closure and involution of the embryonic membranes

In most other Arthropoda (Crustacea, Arachnida, Myriopoda, etc.) development goes on by the formation of a so-called primitive band, but without the appearance of peculiar embryonic membranes. The outer surface of the entire egg becomes, then, in part covered by the band-like embryonic germ, and partly by a portion of the blastoderm which remains unchanged. The dorsal region is formed by the widening and spreading of the primitive band over the greater part of the surface of the egg, while the area of the unchanged section of the blastoderm continually becomes more restricted. It is generally accepted that the latter is concerned in the dorsal closure, because, together with a histological transformation, it becomes involved in the formation of the ectoderm of the primitive band.

A similar form of retrograde structure possibly occurs in the embryos of Poduridæ, in which a dorsal organ has been observed to develop in an early embryonic stage, which bears some relation to the cuticula enveloping the embryo, but whose significance is in general rather obscure.

In most insects the relations are more complicated, since in such cases, the amnion-folds rise on the edges of the primitive band and of the unchanged section of the blastoderm, whose retrograde development is intimately connected with the closure of the back.

A very simple case of dorsal closure, but which certainly is not a primitive one, occurs in Muscidæ and certain other Diptera whose amnion-folds are developed in a rudimentary way. In this case (according to Kowalevsky and Graber), the amnion-folds become smoothed out again. Amnion and serosa become then a simple epithelium, which throughout corresponds to the unmodified type of blastoderm of Crustacea, Arachnida, and Myriopoda, and here seems to share in the formation of the back. More complicated and very manifold relations of dorsal closure and involution of the embryonal membranes occur in other insects, of which Korschelt and Heider distinguish four different types:

1. Involution under the formation of a continuous dorsal amnion-serosa-sac (Odonata).

2. Involution with exclusively dorsal absorption of the amnion (Doryphora).

3. Involution with exclusively dorsal absorption of serosa and separation of the amnion (Chironomus and Trichoptera).

4. Involution with separation of both embryonic membranes (Lepidoptera and Hymenoptera, Hylotoma).

The first type occurs in the most primitive order of winged insects. The second type (Coleoptera) appears to be an independently inherited form of dorsal closure. In the first type, the formation of the amnion-serosa-sac is initiated by a rupture of the two fused embryonic membranes. This rupture in the ventral middle line occurs in Odonata only in the region of the head-section. In the second type only the amnion, in the third only the serosa are concerned in this rupture, while in the fourth type both membranes remain intact until the slipping out of the larva. (Korschelt and Heider.)

j. Formation of the germ-layers

The older views on the structure of the layers of the primitive band of insects were thoroughly unsatisfactory. Bütschli first found that in the bee, by a kind of folding process, an inner layer of the primitive band arose. Soon afterwards Kowalevsky, by the employment of section-cutting and thorough researches, laid the foundation of a more exact knowledge of these layers. He found that in Hydrophilus a furrow extended along the whole length of the primitive band (Fig. 515, A, B, r), which, while invaginating or sinking in, gave rise to the inner layer of the primitive band, i.e. the common rudiment of endoderm and mesoderm (Fig. 539, A-C).

Kowalevsky also found similar conditions in the honey-bee (Apis), Lepidoptera, and other forms. The furrow above mentioned must be regarded as a very long gastrula invagination, extending along the entire ventral side of the embryo, and the edges of the furrow as a long-drawnout blastopore. The tube arising in Hydrophilus through the closing of the furrow we may regard as a primitive intestinal canal.

The first rudiment of the gastrula furrow appears in insects as two folds extending along both sides of the median line in the thickened ventral plate (Fig. 536, f), through whose formation a more median section of the ventral plate, the so-called middle plate (m), becomes separated from the side plates (s). As the middle plate curves in and becomes overgrown by the folds forming the edges of the blastopore, the gastrula-tube (Fig. 539, A, r) is formed, and furnishes the rudiments of the lower (inner) layer. The ectoderm, then, according to Heider, arises from the lateral plates of the primitive band. The growth of the edges of the blastopore, by which the closure of the gastrula-tube is effected, takes place latest in the region of the most anterior part of the furrow (Fig. 515, B and C), corresponding to that place in the primitive band in which the stomodæum afterwards develops.

During the invagination of the middle plate and its transformation into the gastrula-tube a change takes place in its histological character (Fig. 539, A and B). While it originally consists of a high cylinder epithelium, which after farther changes becomes divided into several layers, since the wedge-shaped single cells push themselves over each other, the cells in later stages become more and more cubical or irregularly polygonal (Fig. 539, B), and are irregularly arranged. At the same time the gastrula-tube is compressed in a dorso-ventral direction. While it in this way spreads out laterally under the side plates (ectoderm), its originally circular primitive lumen passes into the form of a horizontal fissure, which in Hydrophilus long remains as the boundary between the two layers of the inner (or lower) membrane. (Korschelt and Heider.)

There are numerous variations of the process of gastrulation, which are by Korschelt and Heider divided into three types, as follows:—

1. Through invagination and formation of a tube (Fig. 539, A, Hydrophilus, Musca, Pyrrhocoris, etc.).

2. By a lateral overgrowth (Fig. 537, Lepidoptera and Hymenoptera).

3. By an inward growth of cells from a median furrow (Aphides and Trichoptera).

In Doryphora and Lina (Fig. 524) the hinder end of the gastrula furrow is forked.

The cellular layer arising from the gastrula invagination (lower layer) forms the common germ of the endoderm and mesoderm. It has only recently become known how these two germ-layers of insects have become differentiated. Kowalevsky first discovered in Musca that the greatest part of the lower (inner) layer yielded mesoderm exclusively, and that a cell-mass only corresponding to the most anterior and posterior end of the primitive band was used in the formation of the endoderm. We must therefore, in insects, speak of a fore and a hinder endodermal rudiment. In proportion, now, as the ectodermal invaginations, which are destined to form the stomodæum and the proctodæum sink beneath the surface of the embryo, the cell-masses of which the two endodermal rudiments are composed are pushed farther in, and a separation between them and the mesoderm is thus effected. The two endodermal rudiments now form accumulations of cells which lie closely adjacent to the blind ends of the stomodeal and the proctodeal invaginations. They soon widen out into two hour-glass-shaped rudiments, which are directed with their concavities towards each other, but with their convex side towards the nearest pole of the egg. They soon change their form; two lateral stripes grow out from them, and each now assumes the form of a U (Fig. 538, en′). The limbs of the fore and hind U-shaped rudiment are directed toward each other, and grow towards each other until they meet, and are fused together. Thus the endodermal rudiments arising out of the fusion of the two U-shaped rudiments form two stripes extending along the primitive band and situated mostly under the primitive segments. At the two ends the endodermal rudiment fuses with the stomodeal and proctodeal invaginations. These lateral endodermal streaks now spread out, and gradually begin to grow over the yolk, on whose outer surface they lie. This overgrowth makes the greater advance on the ventral side, so that the two endodermal streaks first unite in the ventral median line and afterward in the dorsal. The yolk in this way passes completely into the interior of the rudiment of the mid-intestine.

Kowalevsky has already proved that it is the median parts only of the inner layer which at the two ends of the primitive band become separated as endodermal rudiments through the advance of the stomodeal and proctodeal invaginations: the lateral portions become mesoderm.

Kowalevsky has compared the germ-layers of insects with those of Sagitta. This comparison is supported by the later researches of Heider and of Wheeler on Coleoptera. (See Korschelt and Heider, p. 809.)

Relations somewhat different from the common type of formation of germ-layers occur in Hymenoptera. Kowalevsky and also Grassi agree that here also the endoderm originally forms a part of the lower (inner) layer. But the separation of the endoderm from the mesoderm goes on in Apis in such a way that the two ends of the inner layer pass up to the dorsal side of the egg, where the fore and hind rudiments of the endoderm extending along the back of the embryo grow together. When the two horseshoe-shaped rudiments have met each other and become fused, the enclosing of the yolk begins, which accordingly here proceeds from the dorsal towards the ventral side, instead of vice versa. As a result the endodermal cell-layer in Apis (and also Chalicodoma) at first does not lie under the primitive band, but on the dorsal side of the egg under that flat epithelium, which, arising from the amnion-fold, completes the provisional closure of the back.

The yolk-cells and secondary yolk-segmentation are discussed by Korschelt and Heider at this point. The yolk-cells are elements scattered throughout the yolk and which partly remain in the yolk during the formation of the blastoderm (Fig. 507, C and D), but which in part through a later immigration pass out of the blastoderm into the yolk. Graber has proved the fact of the migration of cells from the lower layer into the yolk, and his observations have been confirmed by other authors. Indeed, in certain cases (Melolontha), these later immigrant cells are clearly distinguishable by their histological characters from those originally found in the yolk.

The yolk-cells are regularly scattered throughout the yolk. Their use to the embryo lies in the fact that they absorb the particles of yolk, which they digest and thus reduce to a fluid condition. It usually happens that after the complete formation of the primitive band there results a delimitation of the areas enclosing each yolk-cell, and this occurrence is called secondary yolk-division. In special cases (Apis, Musca) this occurrence seems not to take place. The yolk-cells are still, after the complete formation of the mid-intestine, to be recognized in the yolk-remnants filling the interior of the same, and gradually become absorbed.

k. Farther development of the mesoderm. Formation of the body-cavity

We have seen that by means of an invagination extending throughout the entire length of the primitive band a layer of cells is produced which soon spreads out on the inner side of the band and thus forms a second lower (inner) layer (Fig. 539, C). From this inner layer is separated at the anterior and posterior ends of the primitive band, the endoderm, which lies in direct contact with the invaginations of the proctodæum and stomodæum. The remainder, by far the most extensive part of the inner layer, is the mesoderm.

The mesoderm now becomes divided into two lateral streaks (mesodermal streaks), by the withdrawal of its cells from the median line (Fig. 539, D). This withdrawal is not, however, always a complete one. In the free median space thus formed, the yolk often forms the so-called median yolk-ridge. Segmentally arranged cavities soon appear in the lateral region of the mesoderm (the primitive segmental cavities), and the bordering mesoderm-cells arrange themselves in the form of an epithelium, and constitute the wall of the primitive segments or cœlom-sac. (Korschelt and Heider).

The primitive segmental cavities in general arise through a split in the mesoderm. In Phyllodromia, according to Heymons, the primitive segments are very extensive. The mesoderm, at the time of the formation of the rudiments of the appendages, is raised with the ectoderm from the surface of the yolk, and in this way there arise in each segment cavities, which, since they are surrounded by mesodermal elements, become the closed cœlom-sacs (Fig. 540, c, c′, c″).

The cœlom-sacs differ in different groups. They are largest in Orthoptera (Phyllodromia), where they take up almost all the cell material of the mesoderm in their formation, and exhibit certain conditions recalling those of Peripatus. The very large primitive segmental cavities, which in Orthoptera also extend into the rudiments of the appendages (Fig. 540, B, ex), in their later stages are, through the formation of a constriction, divided into a dorsal and a ventral half (Fig. 540, B, c′, c″). The ventral portions of these cavities extending into the extremities soon disappear, while the cells of their walls lose their epithelial nature, and group themselves irregularly into a sort of mesenchym. In this tissue, then, arises, partly through a separation among its cells, partly through the elevation of the same from the upper surface of the yolk, the definite body-cavity. The dorsal portions of the primitive segmental cavities remain unchanged a longer time in order to play a rôle in the formation of the intestinal muscular layer, of the heart, pericardial septum, and sexual organs.

In the highest groups of insects (Coleoptera, Lepidoptera, and Hymenoptera) the primitive segments are not so extensively developed (Fig. 539, D-F, us). They here form only relatively small sacs situated in the lateral parts of the primitive band which correspond to the dorsal section of the cœlom-sacs of Orthoptera. The ventral part is here from the very outset replaced by a mesenchym. As a result in these forms also no cœlomic diverticula occur in the rudiments of the extremities.

The definite body-cavity of insects arises entirely independent of the cœlom cavities, and in fact, as Bütschli showed, through the separation of the primitive band from the yolk (Fig. 539, F, l). It appears bounded on the one hand by the surface of the yolk, on the other side by the irregularly arranged mesenchym cells. Originally we can in cross-sections distinguish three separate cavities of the definite body-cavity (in Hydrophilus according to Heider), a median and two larger paired lateral ones which later fuse with each other and with wide lacunæ (e.g. in the appendages) arising by the separation of the mesenchym cells. We refer the compartments of the definite body-cavity, as in Peripatus, to the primary body-cavity or segmentation-cavity. They are only lacunæ in the area of the mesenchym, and throughout bear the character of a pseudocœl.

In later stages of embryonic development the cœlom-sacs and the definite body-cavity enter into communication with one another (Fig. 523, A, us, lh). (Korschelt and Heider.)

Then the hinder cœlom-sacs unite through the degeneration of the transverse dissepiments which separate them. After this a fissure opens in the median wall of the cœlomic sac, through which its cavity unites with the definite body-cavity. In the subsequent changes which the wall of the cœlom-sacs undergoes, these can be recognised no longer as separate divisions of the whole body-cavity.

l. Formation of organs

The nervous system.—As we have already seen (p. 554), the rudiments of the ventral nervous cord arise, after the gastrula invagination is completed, as two ectodermal thickenings situated on each side of the median line, the so-called primitive rolls or strips (Fig. 528, s), which extend from the centre of the procephalic lobes of the head to the last segment, enclosing between them the single median “primitive groove” (Fig. 539, C, pr, and pw).

Soon after the appearance of the primitive strips, the first traces of segmentation may be detected. The ventral cord is from the first in direct connection and continuous with the brain. From the segmental expansions of the primitive strip arise the ventral nervous ganglia, and from the intersegmental constrictions are developed the paired longitudinal commissures.

Transverse sections of the ectoderm in the region of the primitive strips (Figs. 539, C, and 517) show several layers of cells. Of these cellular layers the deeper ones afterwards, by a kind of delamination, separate from the superficial ones and form the “lateral cords,” i.e. the germs of the longitudinal cords of the ventral ganglionic cord. Meanwhile the primitive groove (pr) deepens and forms an invagination extending between the lateral cords. The cells at the bottom of this invagination form the so-called “median cord,” and give rise to the transverse commissures connecting the ganglia.

Wheeler has detected in the rudiment of the ventral cord of several Orthoptera, on the upper surface of the lateral cords, four large cells which he calls neuroblasts (Figure 541, n₁-n₄), from which cells arise by budding and become arranged in vertically arranged layers or pillars (z). Graber has observed them in Stenobothrus and Viallanes in Mantis. These neuroblasts are only present in the inter-ganglionic region, and soon move back to the hinder side of the transverse commissures.

At first there is a pair of ganglia to each of the 16 trunk-segments of the embryo, but afterwards these become more or less fused together; thus those of the three gnathal segments unite to form the subœsophageal ganglion of the adult, and the last abdominal ganglia are fused together and move a little anteriorly (see also pp. 227, 228).

Development of the brain.—The supraœsophageal ganglion is due to the spreading out of the procephalic lobes. The rudiment of the brain is due to a thickening of the ectoderm on the sides of the mouth and of the forehead, this expansion of germinal brain-cells being the direct continuation of the primitive rolls or strips, and which finally becomes differentiated into the protocerebrum, deutocerebrum, and tritocerebrum, as stated on p. 228.

The ganglion opticum, now regarded as a part of the compound eye, arises as an ectodermal thickening on each side of the rudimentary brain. The optic ganglion belongs exclusively to the foremost division of the brain (see also p. 227).

Development of the eyes.—Compound eyes do not appear until the beginning of pupal life, the single eye (ocellus) being the primitive organ of vision. The ocellus of Acilius, according to Patten, arises as a pit or depression of the ectoderm (Fig. 542). The long hypodermal cells which form the walls of this pit or hollow are arranged in a single layer, and bear at their free ends a striated cuticular edge (c), while from their inner or basal end arise the fibres destined to form the common optic nerve.

At a later stage (Fig. 542, B), the eye-pit is closed over, the edges growing over and covering the deeper part of the eye. In this way there arises out of the pit-like rudiment a two-layered optic cup. The outer or superficial layer (l) becomes in its central part the crystalline lens, while the peripheral parts form the iris. From the cuticular striated border of these cells arise the chitinous or corneal lens. On its outer edge the superficial layer of the eye passes gradually into the unmodified hypodermis (h).

The inner, deeper layer of the eye, which forms the contracted cup-shaped portion, appears to be the rudimentary retina (r). From its cuticular rod-like or fibrous edge arise the visual rods. There soon arise certain peculiarities characteristic of the eye of Acilius, i.e. the fissure (sp) bordered by the horizontally situated rods of the large retina-cells (x).

In the farther developed eye (Fig. 543) there is a flattening of the cup-shaped inner edge, by which the bottom of the eye is levelled and the little rods belonging to it stand up vertically (Fig. 543, B, st). Then the cells belonging to the edge of the retinal cup (m) are turned in, forming an inverted layer constituting the germs of a third layer interpolated between the two chief layers of the eye. (Korschelt and Heider, from Patten.) Patten concludes that the structure of the retina in the larval ocelli of insects is much like that of myriopods, and that the whole eye is constructed on the same plan as that of Peripatus and most molluscs.

Intestinal canal and glands.—The intestinal or digestive canal is primitively divided, as already stated on p. 299, into three sections, of which the anterior and posterior are called respectively the stomodæum and proctodæum, and are invaginations of the ectoderm, forming sacs whose blind ends face the future site of the mid-intestine. The fore-intestine (stomodæum) in most cases arises earlier than the proctodæum. Its muscles are derived from the mesoderm. From the stomodæum arises at an early date an unpaired dorsal invagination out of which develops the ganglion frontale and the pharyngeal nerve.

The absorption of the ends of the blind sacs of the fore and hind intestine, and opening up of the passage into the mid-intestine, occur rather early in embryonic life. In the wasps and bees, as well as the larva of the ant-lion, the mid-intestine remains closed at the end, not communicating with the proctodæum, which has an exclusively excretory function (Fig. 497).

The mid-intestine arises from two originally separate rudiments, i.e. the fore and hind endodermal rudiments, which at the outset stand in the most intimate relation with the invagination of the fore and hind intestine. Originating as a simple collection of cells, so closely adjoining these invaginations that Voeltzkow, Patten, and Graber derived them directly through outgrowths of them, they become extended by advancing cell-multiplication until they assume a U-shaped form. The legs of the U-shaped rudiment are in the anterior endodermal mass, directed backwards; those in the posterior mass, on the other hand, are directed anteriorly. These legs grow towards each other until they become fused together, forming two paired endodermal streaks, which pass under the primitive band along its whole length, and are fused with it at the fore and hind ends. In these places they stand in intimate union with the proctodeal and stomodeal invaginations.

The paired endodermal streaks belong to the lateral portions of the primitive band. As a rule, they lie directly under the row of cœlom-sacs (Fig. 539, F). The dorsal wall of the primitive segments stands consequently in intimate contact with the endodermal streaks. On this wall of the primitive segments an active cell-growth takes place, and the cell-material produced in this way, which separates from the dorsal wall of the primitive segments, forms the outer or splanchnic layer of the rudiment of the mid-intestine (spm, Figs. 539, F, 544, sp). What remains of the dorsal wall of the cœlom-sacs after this separation joins the genital rudiments and gives rise to the so-called terminal thread-plate (Fig. 544, ef). The endodermal streaks, with the splanchnic layer lying next to them, may now be considered as the rudiments of the mid-intestine (Fig. 530, m, etc.). These are noticeable in the following stages by their considerable lateral growth; they spread out over the upper surface of the yolk, around which they finally entirely grow (Figs. 539, C-F, 544, 545). This growth around the yolk goes on in most cases in such a way as to unite the two mid-intestinal streaks in the region of the ventral median line with each other. Then afterwards their union on the dorsal side takes place (Figs. 539, F, 545). The yolk thus passes completely into the interior of the mid-intestine, and with it the remains of the dorsal tube or dorsal organ, when such an one is present.

The salivary glands.—These segmentally arranged glands, which open by pairs into the three gnathal segments of the head, arise as ectodermal invaginations originally opening not into the stomodæum, but outwards on the surface of the body; hence Korschelt and Heider suggest that they were originally dermal glands, whose mouths became drawn into the buccal cavity.